Why is the following comment by Macbeth in Act V, Scene 5 important?
The time has been my senses would have cooled
To hear a night-shriek, and my fell of hair
Would at a dismal treatise rouse and stir
As life were in ’t. I have supped full with horrors.
Direness, familiar to my slaughterous thoughts cannot once start me.
Macbeth seems to be saying that, having seen (and caused) so much horror, that he is now immune to its effects. When he says "Direness, familiar to my slaughterous thoughts, cannot once start me," he means that he is now unmoved by suffering. The first part of this excerpt recalls what we have already seen, namely that Macbeth was not always this way. It was his wife who had to steel him to commit the initial murder of Duncan early in the play. By the end of the play, however, he seems bereft of human feelings. He confirms this with his jaded response to the news of his wife's death:
She should have died hereafter;
There would have been a time for such a word.
Tomorrow, and tomorrow, and tomorrow
Creeps in this petty pace from day to day
To the last syllable of recorded time;
And all our yesterdays have lighted fools
The way to dusty death. Out, out, brief candle!
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more. It is a tale
Told by an idiot, full of sound and fury,
Through this moment of self-reflection and the one immediately before news of his wife's death arrives, Shakespeare demonstrates just how far Macbeth has slipped.
This quote from Act V, Scene V, is important because it demonstrates just how much Macbeth has changed over the course of the play. In this scene, Macbeth hears a woman crying and notes that the sound has absolutely no effect on his emotions or nerves. But he remembers a time when an unexplained noise, just like this one, would have unnerved and frightened him.
Macbeth acknowledges the reason for this change: he has witnessed so many "horrors" that he no longer experiences fear in the way he used to. These "horrors" relate to his pursuit of the Scottish throne. The murder of King Duncan, for example, and of his friend, Banquo, and the family of Macduff. Macbeth has committed so many brutal acts of violence that his reactions are no longer the same. He has become desensitized to brutality.
Macbeth's ambition has, therefore, changed his character beyond all recognition, and this quote demonstrates that he is fully aware of such changes.